Ian in NZ is building one of my Goat Island Skiffs – doing a very nice job.
However, on in detail on my forum, he started talking about making the rather expensive PVC bags you can buy as buoyancy bags or dry bags. Here is a video of Philip Printz going crazy in a prototype narrow Quick Canoe with buoyancy bags in both ends (blue)
I will hand it over to Ian.
It’s possible to make custom buoyancy bags for kayaks and boats quite easily.
The key is having the right glue. The right glue is “heat activated urethane/PVC glue”. Down under it is called Bostik Unigrip 999, in the USA look for HH-66.
Problem is, it doesn’t seem to be available retail down here, good thing is all PVC boat and truck tarpaulin makers will be using it or the equivalent, so go to your local PVC tarpaulin maker/upholsterer and buy half a tin from them.
I wouldn’t worry whether it’s HR or HV unless you’re making an inflatable boat that will stay outside in all weathers.
If you want to know why it’s worth going to the trouble of getting this glue, watch them making the heavy PVC curtain sides for a big curtain sided truck. They sew seatbelt webbing to a patch of PVC with an industrial sewing machine, coat the other side of the PVC patch and the area on the main PVC fabric wiith this glue, let it dry, press the two together, go over it with a hair dryer, and voila, the patch is stuck on strongly enough to take tensioning down with one of those ratchet tensioning thingymies they use.
Inflation valves are easy, just get a couple of cheap swim wings and cut out the valves.
You might be able to get bigger ones, or extra screw on/off caps that let you deflate the bags more quickly, by buying/stealing and cutting up a cheap kids paddling pool (WAAAAAAAH – Ed). might even be possible to buy these sort of fittings, but the swim wings are cheap and provide 4 valves a set.
PVC: 600g/m^2 (18oz/square yard?) or so is probably best, but you could go lightweight at 400g/m^2 or extra heavy duty at 1000g/m^2.
Cut out a rectangle, as long as you want the bag plus a little bit, width about 3 and a bit times the diameter you want plus a little bit extra (think Pi).
Decide where you want the valve, cut an appropriately sized hole there. Coat the nipple side of the material around the nipple, and the inside of the fabric around the hole. Let the glue dry, press the nipple through the hole, put it on a surface nipple up, then apply heat with a hair dryer.
If you’re the paranoid type, cut another slightly bigger circular patch of PVC with a hole in it and apply it in the same way to the inside.
Next, mask about 20mm inside from the edge of the fabric rectangle all the way around, apply glue, let it dry, fold the rectangle in half so glued surface meets glued surface, work your way around the edge, pressing the surfaces together and applying heat with the hair dryer till it’s all glued together.
Make a disposable brush from a wooden icecream stick and a piece of loop velcro folded over the end and stapled on. Main thing to avoid is thick goopy lumps of glue – try and get the spread reasonably even, and make sure the solvent is completedly evaporated before you apply any heat.
Tom Yost has quite a bit of information on working with PVC on his Folding Kayak Builders website. Manufacturers websites and Tom’s site mention taking lots of care cleaning the PVC with Methyl Ethyl Keytone (Nasty solvent, used for making methamphetamine amongst other things).
Advice I have is don’t bother, the glue sticks well anyway and solvent is more likely to weaken the fabric around the bonded area. When the glue is stuck, the two layers where it’s bonded are stronger than the surrounding fabric. Make up a football sized one first, inflate it and give it to the kids to kick around the yard. If it’s still inflated a couple of days later, you can have confidence in your construction technique.
While you’re at it, make a couple of long narrow tubes, inflate them and give them to the kids to beat each other over the head with while they run up and down the hall.
It is important that bags are tied down well in the boat, it’s only the part that is displacing water that is contributing to buoyancy. I once saw an 18ft fibreglass sloop with a steel swing keel go right to the bottom when it swamped, the buoyancy bags ripped out one by one. The buoyancy bags (and the skipper/crew) stayed on the surface but the boat didn’t. One embarrassed sailing instructor (“I was only teaching them capsize recovery”) when we picked them up.
I’d take the bags out of the boat between trips and store them inside (not wet, not in the sun, not somewhere where the PVC can freeze) With a bit of care they should last 3 seasons, I’d replace them after that.
Once you’ve tried a couple, have a go at custom shapes if it suits. If you need to attach straps to the bags, sew the straps to a piece of PVC, then glue it to the bag before you glue up the final seam around the outside. It’s possible to make bags with an opening that rolls up with velcro that holds the rolled seal shut, then you can combine gear storage and buoyancy.
And here is the dry bag method
Here is a pattern to make dry bags similar to the commercial ones.
You need PVC material 400 or 600g/sqm, glue (read comments in the first post regarding glue), a quick release friction buckle .
Make up the strap that attaches the buckle first, easiest is to cut out a strip of heavier PVC and glue it. (Raid the scrap bin at a PVC tarp maker, they will have lots of long narrow scraps of heavier material)
Next, glue the strap to the other side of the main material, turn it over, mask and then glue all the way down both sides of the main material, take the masking tape off.
Once the glue is dry, fold the bottom half up, match up the glue lines, apply heat with a hair dryer and it’s all done.
I also tried some out of light PVC coated nylon, I’d stick to the heavier PVC for ones tied down in the bilge of a boat, but the very lightweight ones held up surprisingly well for backpacking or for a sleeping bag that was going to stay in what should be a dry boat storage compartment.
You could get fancier and do a round bottom like the commercial ones, but this way works fine.
A couple of other tips … seams are 15 to 20 mm wide. The surface needs to be brushed with solvent and wiped off with a clean cloth before gluing – the Bostick #4 solvent is the right one, but others might work. The glue must be let dry on both surfaces before bonding. It is heat activated so you align the glued surfaces together then warm the area with a heat gun and then press down – not too hard using a rag to protect your hand from the heat – in various places on the net I have seen the heat from a heat gun only takes 2 or 3 seconds to heat the PVC enough. I would probably do some trials on scrap first to get the hang of it. You can pull them apart to make sure your method is working well!
Nothing like a bit of destructive testing!
The original Goat Island Skiff builder in the USA, David Graybeal, had this link
Brian Pearson in the UK found these suppliers in the UK
The HH-66 has never been available in the UK before now, but that has now changed and HH-66 is now available here. I don’t promise it is the cheapest supply, but that is the packaging and product.
Here are a couple of PVc materials suppliers The Protective Textile Co Ltd PVC Sheet, PU Nylon, Nylon, Polyester reinforced pvc
The curiously named “Titch the Clown had this supplier in Australia
The Australian (rather than Kiwi) equivalent is Bostik 1669 Heat-Activated Contact Adhesive see Vinyl supplies in Melbourne for SOF kayak?
If there are any addtions to this discussion you can see the whole thing here on my forum